This invention relates generally to multi-band television channel selection, and more particularly relates to automatically generating appropriate bandswitching signals upon channel number selection for tuning a varactor-controlled, resonant frequency, tunable circuit to all VHF, UHF and CATV channels.
Electronic tuners are replacing the more conventional electro-mechanical tuners in current production television receivers at an increasingly high rate because of their improved performance, reduced cost, higher reliability and more appealing user interface characteristics. These electronic tuners typically incorporate several combined tuned circuits the resonant frequency of which is continuously variable by means of a varactor diode having a variable capacitance which varies as a function of an applied DC voltage. The operating frequency of these coupled tuned circuits may thus be adjusted over a given frequency spectrum to permit reception by the television receiver of all signals transmitted therein. However, today there are three separate spectra over which television signals may be received, i.e., the VHF, UHF, and CATV spectra. The VHF television spectrum embraces channels 2-13 covering the frequency range of 57-213 MHz. The UHF spectrum includes channels 14-82 covering the frequency range of 460-910 MHz. The CATV allocated channels are in a so-called Midband consisting of channels A-I (123 MHz. to 171 MHz.) and a so-called Superband consisting of channels J-Z (219 MHz. to 315 MHz.). Recently, these letter-designated channels have also been given numerical designations with channel A-I being channels 83-91 and channels J-Q being channels 92-99. Channel A has been abandoned and it is almost certain that channels R-Z will be reallocated. Thus, there are currently approximately 112 channels available for the transmission of television signals. From the above discussion it can also be seen that these various television channels cover a broad frequency spectrum which has resulted in demanding performance criteria being placed on the design and development of a signal tuner capable of tuning in all currently available television channels.
One difficulty in this area has involved bandswitching to permit the tuned circuits to cover several transmission frequency bands. Providing a VHF and UHF tuning capability in a signal tuner has been accomplished by a number of different approaches. One VHF/UHF tuner control system approach is disclosed in U.S. Pat. No. 3,942,122 to Nakanishi wherein is described a conventional 3-band varactor VHF tuner in which one of the three VHF tuning band circuits is modified to accommodate a UHF IF band generated for tuning to UHF channels. This tuner control system is limited to coverage of only the VHF and UHF bands and involves the use of ten UHF and four VHF potentiometers which reduces system tuning accuracy and severely limits reliability.
Prior art approaches to providing a television receiver with the capability of tuning to VHF, UHF and CATV channels have suffered from various limitations. Typically these attempts involved the use of an accessory known as a frequency converter to provide for the reception of CATV channels. This converter, typically, converts the received CATV signals to a common IF frequency which is generally that of VHF channel number 4. The cable television signals are then applied to a selected channel's position on a conventional VHF tuner. The converter accessory is either incorporated within the tuner housing or supplements the tuner as an additional component located exterior to the television receiver. The need for this converter has resulted in all-channel tuning systems being expensive, unwieldly and overly complex because of the high frequency switching required therein. The presence of this accessory converter also precludes the use of a remote control channel selector over the VHF, UHF and CATV bands.
The basic limitation in prior attempts to develop a single VHF/UHF/CATV channel selection system is related to the capacitance variation in the tuned circuits and the resulting limited frequency spectrum to which these resonant circuits could be tuned. Of course, by increasing the capacitance of the varactor diode in the tuned circuit the frequencies available may be correspondingly increased. However, the internal resistance of the varactor diode increases for higher rated breakdown voltages. Thus, while a greater capacitance-voltage range is obtainable by using diodes of higher voltage breakdown and increasing the tuning voltage, varactor diode internal resistance limits attainable gain in the tuner at the high frequency channels and is, as a result, counter productive. This characteristic of varactor diodes imposes an upper limit on the tuning voltage of approximately 25 volts. While this is a practical voltage range for a VHF band tuner, and even for a combination VHF/UHF tuner, it is not possible to tune to all VHF, UHF and CATV channels within this limited voltage range because signal levels in the tuner are of sufficient magnitude to require inter-channel tuning voltage separation which cannot be accommodated for all channels in these bands within the 25 volt limitation. Despite these limitations, various attempts to provide a multi-band tuner are disclosed in the prior art.
One approach is disclosed in U.S. Pat. No. 4,161,708 to Friberg et al involving the division of the frequency spectrum into different band segments permitting a reduction in the capacitance range required of the varactor diode. By thus reducing the range through which the varactor is required to tune, large impedance variations normally present as the resonant circuit is tuned across the entire reception band are reduced. The frequency ratio of the entire reception band is thus reduced from nearly 3 to 1 to only a ratio of 1.44 to 1 for each of the individual band segments thus produced. While the Friberg approach offers an efficient means to tune over a given RF spectrum, it is specifically intended and designed for radio signal reception, and more specifically for automobile radio reception, and thus would not operate at the higher frequencies of television signals. For example, the Friberg invention is intended to operate over a reception band of 540-1600 KHz, while the VHF, UHF and CATV spectra encompass a frequency range measured in MHz. U.S. Pat. No. 4,002,986 to Ma discloses a system for receiving VHF, UHF and CATV signals involving a plurality of varactor diode tunable stages each having a capacitance range variation as a function of applied tuning voltage which is insufficient to enable the tuner to receive television signals in all three frequencies bands. Consequently, bandswitch means are provided for adjusting the inductive reactances of these tunable stages to permit tuning to more than one television signal in different frequency bands with substantially the same tuning voltage value. This approach suffers from the limitation that a low frequency DC switch must be manually positioned in order to receive either VHF or CATV channels while in the VHF reception mode. Thus, the convenience of multi-band reception simply by selecting a particular channel number is not available.
Still another approach to a multi-band tuner is disclosed in U.S Pat. No. 4,118,679 to Hiday et al which discloses a combination VHF, UHF and Superband tuner which provides automatic bandswitching by activating or deactivating voltage-sensitive switching devices such as switching diodes to produce the desired circuit connections. While this approach does provide the advantages of electronic bandswitching with the automatic generation of bandswitching voltage upon channel number selection, it includes a manual three position (VHF, UHF and Superband) switch with its inherent inconvenience to the user and relatively low reliability mechanical configuration. This band selector switch provides a B+ source of energizing voltage to the appropriate tuner. Finally, mention is made of U.S. Pat. No. 4,015,192 to Koyanagi because it discloses a voltage generation system useful in producing a channel selection voltage for an electronic tuner and which is capable of generating a large range of different voltages to be applied to a varactor diode-type tuner. This invention involves the use of a plurality of variable resistances in parallel which are individually and selectively coupled to and decoupled from the voltage generating circuit by means of switching transistors through which the input control signals are provided to the voltage generating circuit. In this manner, a large variety of resistance values may be incorporated in the circuit to produce a large range of output voltages. While this invention is useful in tuning voltage generation, it fails to offer any bandswitching capability for a varactor-type tuner.
In accordance with the present invention, all of the aforementioned limitations are avoided by means of a single, all-electronic, channel tuning frequency synthesis system capable of operating over the entire VHF, UHF and CATV spectra in which digital bandswitching is accomplished automatically with channel number selection. Digital bandswitching and tuning voltage signals are provided by a microcomputer to bandswitching circuitry and a phase-locked loop, respectively. Appropriate signal level shifting is accomplished between the microcomputer and the tuner to provide proper B+ and bandswitch signals to a varactor-type tuner.